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					The 2 Stroke Crosshead
• The 2 stroke diesel crosshead engine works on exactly
  the same principle and cycle as the 2 stroke diesel trunk
  piston engine.
• The disadvantages of the two stroke diesel trunk piston
  engine are that although it has a low overall height,
  lubricating oil splashed up from the crankcase to
  lubricate the liner can find its way into the scavenge
  space, causing fouling and a risk of a scavenge fire.
  There is also the likelihood of liner and piston skirt wear,
  allowing air into the crankcase. This can supply the
  required oxygen for a crankcase explosion should a hot
  spot develop. The crankcase oil must have additives
  which can cope with contamination from products of
  combustion, and the acids formed during combustion
  due to the sulphur in the fuel.
• The majority of 2 stroke
  engines encountered at
  sea are of the "crosshead"
  type. In this type of engine
  the combustion space
  (formed by the cylinder
  liner, piston and cylinder
  head), and the scavenge
  space are separated from
  the crankcase by the
  diaphragm plate.
• The piston rod is bolted to the piston and passes
  through a stuffing box mounted in the diaphragm
  plate. The stuffing box provides a seal between
  the two spaces, stopping oil from being carried
  up to the scavenge space, and scavenge air
  leaking into the crankcase.
• The foot of the piston rod is bolted to the
  crosshead pin. The top end of the connecting
  rod swings about the cosshead pin, as the
  downward load from the expanding gas applies
  a turning force to the crankshaft.
• To ensure that the crosshead reciprocates in alignment with the
  piston in the cylinder, guide shoes are attached either side of the
  crosshead pin. These shoes are lined with white metal, a bearing
  material and they reciprocate against the crosshead guides, which
  are bolted to the frame of the engine. The crosshead guides are
  located inbetween each cylinder.
• Using the crosshead design of engine allows engines to be built with
  very long strokes - which means the engine can burn a greater
  quantity of fuel/stroke and develop more power. The fuel used can
  be of a lower grade than that used in a trunk piston engine, with a
  higher sulphur content, whilst high alkalinity cylinder oils with a
  different specification to that of the crankcase oil are used to
  lubricate the cylinder liner and piston rings and combat the effects of
  acid attack.
• The most powerful diesel engines in the
  world are two stroke crosshead engines.
  Some of these engines have cylinder
  bores approaching 1metre with a stroke of
  over 2.5 metres. The crankshaft can weigh
  over 300 tons, with the engine weighing in
  excess of 2000 tons.